This invention relates to electric heating elements, for example for use in liquid heating vessels such as kettles, rice cookers, coffee makers, etc.
Various types of heating elements are known for the above applications. Until recently, the most common form of heating element was the immersion element which basically comprises a metal coil which heats up when a current is passed through it.
Recently, there has been a tendency to produce flat heating elements. In the case of electric kettles, for example, these provide the advantage that cleaning the inside of a kettle is easier, and it may be possible to boil a smaller quantity of water, since a smaller quantity is required to cover the heating element. A flat heating element also provides improved appearance of the inside of the kettle.
One way to produce flat heating elements is simply to bond a conventional coil to the underside of a flat metal substrate. However, the use of thick film heating elements is now being explored for liquid heating applications. The invention concerns these thick film heating elements.
Conventionally, a thick film heating element comprises a planar metal substrate onto which an insulating dielectric layer is deposited, and a thick film heating track is patterned over the dielectric insulating layer. One problem associated with thick film heating elements is that difficulties arise in obtaining sufficiently high breakdown voltages for the heating elements. These are required so that the heating elements comply with the relevant industry standards. For example, the element may be subjected to a high voltage "flash test" before and after thermal tests, such as a boil-dry and a dry-boil test. During operation of the heating element, in the thermal tests described above as well as in normal operation of the element, thermal stresses cause micro-cracks to occur in the insulating layer. These cracks destroy the dielectric properties of the insulating layer and lead to reduced breakdown voltages. Efforts have been made to increase the breakdown voltage by improving the quality of the insulating layer.